Repeated failures of motor operated valves (MOV) in nuclear power facilities to perform their intended safety functions brought about the issuance of NRC Generic Letter 89-10 (GL 89-10), requiring, among other things, that the facilities demonstrate their safety related MOV's to be operable by testing of the design basis differential pressure and/or flow by which the MOV's are required to perform their intended safety function.
Over the last several years, various manufacturers have developed diagnostic test equipment to assist the nuclear power facilities in quantifying the performance characteristics of MOV's. It soon became evident that valves often produce unpredicted and unexpected results that were in disagreement with the classical valve design equations of the past. In addition, a phenomenon termed "load sensitive behavior" (also known as "rate of loading") was often found to be a problem for MOV's that must close against flow. The root cause for this phenomenon has defied detection for years.
Load sensitive behavior is evidenced as a change in MOV performance between static conditions (no flow) and dynamic conditions (differential pressure and/or flow). The change in behavior is most often noticed as the difference in the thrust produced at a given torque switch setting in closing the valve during static versus dynamic conditions. The difference in thrust is easily detected with the use of modern diagnostic test equipment.
An MOV is usually set up during static conditions with the use of diagnostic test equipment. The valve is stroked, and the closed torque switch set point is set to receive the thrust expected to be required to close the valve disk, plus an additional amount of thrust for safety margin. In order to support GL 89-10 requirements, the MOV R is then stroked under dynamic conditions (flow) to demonstrate that it can perform its intended function.
After dynamic testing, the values of thrust and torque at torque switch trip may be easily compared. Many times, the thrust in the stem at torque switch trip during dynamic conditions is found to be less than the thrust that was observed during static conditions. However, the value of torque is the same. Sometimes the stem thrust under dynamic conditions is found to be greater than the thrust that was observed during static testing. Sets of "identical" MOVs have been found to have very different load sensitive behavior characteristics. Consequently, load sensitive behavior can have tremendous impact on the ability of an MOV to perform its intended safety function.
There is therefore a need for making an MOV behave predictably under static and dynamic conditions.